1. Field of the Invention
The present invention relates to a combustor system and in particular to a combustor system including premixed burners.
2. Prior Art
Conventional low NOx combustors for gas turbine perform only diffusion combustion having a wide range of combustible air/fuel ratio on starting and perform premixed combustion in the course of combustion to achieve reduction in NOx omission as described in Japanese Unexamined Patent Publication Takkai-Sho 59-143852.
Recently environmental pollution problems have been taken up. Demands for low NOx combustors have become stronger. In order to meet these demands, it is necessary to increase the ratio of the premixed combustion to the diffusion combustion.
In order to prevent blow off or backfire, etc. from occurring, it is necessary to keep the speed of premixed gas flow at a burner injection slot within a predetermined range. Accordingly, it is necessary to increase a premixed burner injection slot in size to increase the flow rate of the premixed gas for increasing the ratio of the premixed combustion to the diffusion combustion. If the premixed burner injection slot is merely increased, non-uniform distribution of the concentration of the premixed gas may occur due to non-uniformity of the flow speed distribution. Combustion at higher temperatures may occur where the concentration is higher, i.e. the fuel is rich. This results in an increase in NOx. Specifically, some data show that the area of the injection slot is doubled, the non-uniformity in concentration increases by about 10%, with the result that NOx omission is doubled.
Therefore, a combustor in which premixed burners are provided upstream and downstream of the combustor to adjust the flow rate of fuel for each premixed burner in response to a load has been proposed as described in, for example, Japanese Unexamined Patent Publication Tokkai-Sho 62-267529. Although the total area of the injection slots of the premixed burners is increased, the fuel is uniformly mixed with combustion air without increasing the injection slot area of each premixed burner.
Since the premixed gas which is injected from a downstream premixed burner is directly fired with an upstream premixed flame in such a prior art although the premixed flame is instinctively unstable, the upstream premixed flame is directly cooled and becomes unstable, which at most may result in the quenching of a flame.